This document summarizes the development of a new dispensing platform equipped with an advanced parallel dispensing print head that can process solar cells at high speeds up to 700 mm/s. The dispensing technology allows for substantially narrower finger widths of just 48% of the printed width after encapsulation, leading to almost 50% lower shading losses compared to screen printed cells. Recent cell results using industrial emitters achieved efficiencies up to 0.4% absolute higher than with standard screen printing, reaching 20.5% efficiency on PERC structures. Key advantages are the ability to use screen printing pastes after rheological adaptation and the extraction of benefits of non-contact printing such as very homogeneous finger geometries.

1. Available online at www.sciencedirect.com
ScienceDirect
Energy Procedia 00 (2015) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2015 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of Gunnar Schubert, Guy Beaucarne and Jaap Hoornstra.
5th Workshop on Metallization of Crystalline Silicon Solar Cells
Dispensing Technology on the Route
to an Industrial Metallization Process
Maximilian Pospischila
*, Martin Kuchlera
, Markus Klawittera
, Carlos Rodrígueza
,
Milan Padillaa
, Raphael Efingera
, Michael Linsea
, Angel Padillaa
, Harald Gentischera
,
Markus Königb
, Matthias Hörteisb
, Lars Wendec
, Oliver Dolld
, Roland Zengerlee
,
Florian Clementa
and Daniel Biroa
a
Fraunhofer Institute for Solar Energy Systems (ISE), Heidenhofstr. 2, D-79110 Freiburg, Germany
b
Heraeus Precious Metals GmbH & Co. KG, Electronic Materials Division, Business Unit Photovoltaics, Heraeusstr. 12-14, D-63450 Hanau,
Germany
c
ASYS Automatisierungssysteme GmbH, Solar & New Technologies, Benzstr. 10, D-89160 Dornstadt, Germany
d
Merck KGaA, Performance Materials Division, Postcode Q004/001, Frankfurter Str. 250, D-64293 Darmstadt, Germany
e
Department of Microsystems Engineering – IMTEK, University of Freiburg, Georges-Köhler-Allee 103, D-79110 Freiburg, Germany
Abstract
This study presents a new developed, inline applicable dispensing platform that is equipped with an advanced version of
previously introduced parallel dispensing print heads and works as drop-in-replacement in existing manufacturing lines. At
process speeds of up to 700 mm•s-1 and a substantially improved process stability, the impact of the resulting contact geometries
on optical and ohmic losses was analysed in detail. A reduced finger width as well as an effective width of just 48% after
encapsulation of the finger width leads to nearly 50% reduction of shading losses compared to screen printed samples. A
substantially improved finger homogeneity leads to similar grid resistances at 20% less silver consumption.
Consequently, recent cell results on industrial emitters (Rsh = 90 Ω/sq.) showed an efficiency increase of up to +0.4%abs. in
comparison to standard single screen printing reaching top values of η = 20.5% on PERC structures. A key improvement of the
technology is the new ability to process certain metal pastes originally designed for screen printing applications and thus keep in
track with fast emerging paste development. Successfully evaluated screen printing pastes then can be rheologically adapted in
order to reach ultrafine contact fingers at high aspect ratios and extract the whole advantage of this non-contacting printing
technology.
© 2014 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the scientific committee of the SiliconPV 2014 conference.
* Corresponding author. Tel.: +49 761 4588 5268
E-mail address: maximilian.pospischil@ise.fraunhofer.de

2. 2 Pospischil et al. / Energy Procedia 00 (2015) 000–000
Keywords: silicon, solar cells, printing, metallization, dispensing, contact geometries, economic evaluation, analytic simulation
1. Introduction
Silicon solar cell metallization is still dominated by screen printing technology offering a robust contact formation
and proven long term stability. Forced by a rapid decline of retail module prices, Ag-paste consumption had to be
substantially reduced [1]. Consequently, paste and process development were pushed to achieve printed line widths
of less than 50µm. However, known issues like mesh marks, line spreading or screen wear have not yet overcome.
Different thick film technologies like stencil printing [2] or recently introduced co-extrusion approach [3] are
promising technologies but are challenged by high consumable costs in the former or the competition of fast
emerging screen printing pastes that cannot be directly used in the latter approach.
The dispensing technology, as described by Specht et al. [4], offers a contactless, high-throughput single-step
metallization process significantly reducing finger width and thus shading losses. Record cell efficiencies of 20.6%
on 125x125mm² FZ p-type material using dispensing technology on MWT-PERC (Metal Wrap Through –
Passivated Emitter and Rear Cell) solar cells, featuring a selective emitter structure were presented by Lohmüller et
al. [5]. Dispensing pastes are directly derived from screen printing paste development [6] and resulting finger
geometries can be varied in a wide range by adapting paste rheology as described in previous studies e.g. [7]. In
order to demonstrate the benefit of these advantages, the analytical 2D simulative tool Gridmaster [8] was enhanced
to observe the effect of various geometrical parameters on solar cell results and manufacturing costs [9]. Both
however, imply a stable metallization process at high throughput rates. For this reason, a novel dispensing platform
was developed, providing fully automated inline production feasibility. In the following, this platform, was equipped
with an advanced parallel dispensing print head as introduced in [10] and applied for extensive solar cell processing.
Here, a focus was put on process stability using latest industrially manufactured dispensing pastes with sufficient
contacting behavior on state of the art emitters (Rsh > 90 Ω/sq.).
Nomenclature
Af (µm²) finger cross-section area Rsh (Ω/sq.) Emitter sheet resistance
D (µm) nozzle outlet diameter Voc (mV) open circuit voltage
EW (%) relative effective finger width wBB (µm) busbar width
FF (%) fill factor wc(µm) contact width
jsc (mA/cm²) short circuit current weff (µm) absolute effective finger width
N (-) Number of contact fingers wo (µm) optical (= max.) finger width
rBulk (Ω∙cm) bulk resistivity ws(µm)
screen opening width for single and
double screen printed reference samples
rf (µ Ω∙cm) finger resistivity ρc (mΩ∙cm²) contact resistivity
RGrid (Ω/m) finger grid resistivity  (%) solar cell conversion efficiency
2. Approach
The focus of three years lasting research project “GECKO” was mainly the development of dispensing
technology aiming on an industrial implementation. Here, precise rheological analysis of dispensing pastes [7, 10]
and a subsequent implementation of a rheological paste model allowed for an efficient development of parallel print
heads by computational fluid dynamic (CFD) simulations [10, 11].
After providing a homogeneous mass flow distribution to all nozzles and optimizing the shape of the
incorporated nozzle plates, record finger widths of just 27µm were demonstrated using previous paste generations
[10]. During this study, development was focused on providing high process stability with new developed silver

3. paste
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ses allowing
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4. 4 Pospischil et al. / Energy Procedia 00 (2015) 000–000
3.2. Development of multi nozzle print heads
In order to significantly increase throughput rates during dispensing, a novel parallel print head was developed.
For this reason, rheological paste characteristics were used to implement an universal paste model including Non-
Newtonian flow patterns like shear thinning and yield stress [10]. After verifying this paste model using a single
nozzle dispense setup, the focus was put on the development of a ten nozzle parallel dispensing prototype (Fig. 3).
Here, a modular setup allowed for a separate optimization of dispensing nozzles, valves and paste distribution.
In the following, multi nozzle print head designs were tested and optimized regarding their robustness concerning
fabrication tolerances. Due to specially designed nozzles, the necessary dispensing pressure was reduced by a factor
of up to ten compared to commercial standard nozzles. A central fed paste supply with nozzle pitches of only
1.56 mm was realized which allows for further scalability of the design in the future.
3.3. Integration into an inline applicable platform
Finally, the novel print head was integrated in a newly developed fully automated dispensing platform (Fig. 4)
which permits industrial manufacturing sequences with the new setup. Here, a fully automated cell handling system
allows for a precise application of dispensed grid structures on industrial solar cells.
Continuous line dispensing at line speeds of more than 700 mm•s-1
was already reached using this setup.
Furthermore, a nozzle distance of just 50 µm can be realized during dispensing with this platform. However,
rheological adaptions of the dispensing pastes also allow for a stable process at much greater distances by stretching
the paste during its free flow phase.
3.4. Concepts for Inline Integration
Most standard back-end production line concepts consist of three subsequent printing steps. Two are needed for
the back surfaces, namely printing of Aluminum providing a full area contact of the p-type base and local silver pads
that are required for soldered module interconnection. The front side grid is finally printed using a third screen
printer which is followed by the fast firing oven (FFO) as visible in Fig. 5, top route. In order to further increase cell
efficiencies, various routes with four printers in series have been introduced, mainly dual and double printing. The
former allows for a reduction of recombination losses and silver consumption by applying a specially designed, non-
contacting busbar paste (i.e. floating busbar concept [12]). Here, contact fingers are alternatively applied by stencil
printing. The latter (i.e. double printing or print on print) increases finger aspect ratio (AR) and reduces the
influence of mesh marks by adding a second printing step that is precisely aligned to the first one, see Fig. 5, second
route.
The integration of the inline dispensing platform states the third route in Fig. 5. Here, a conventional busbar
printing step is followed by the application of the dispensed finger grid before entering the FFO. In opposite to all
previous routes, the contact less dispensing step does not require a previous drying step which allows for a further
reduction of investment costs and foot print of the tool. With all other back end steps remaining the same, the
integration of a dispensing unit can be seen as drop-in-replacement compared to all other dual or double printing
approaches. With line speeds of more than 700 mm•s-1
, the expected through put of the back-end production line by
far is not limited by the dispensing process. Moreover, even a junction of two parallel printing lines into one
dispensing unit is possible providing that the print head contains a sufficient number of parallel operating dispensing
nozzles.

5. Fig. 3
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6. 6 Pospischil et al. / Energy Procedia 00 (2015) 000–000
However, another advantage of the dispensing process is its substantially increased finger homogeneity (Fig. 7)
that allows to print extremely fine lines without drawbacks coming from mesh marks or paste spreading that both
lead to additional silver consumption that does not contribute to lateral current transport within the fingers. In order
to illustrate this effect, wet paste consumption of various printing experiments on industrial Cz material with both
technologies was plotted against the values of the grid resistance RGrid (Fig. 8) obtained from IV-measurement of the
corresponding solar cells.
Fig. 7 Comprison of finger homogeneity of the applied technologies by
means of the standard deviation in finger cross section area Af.
Fig. 8 Correlation of wet paste consumption and grid resistance RGrid
for dispensing and screen printed solar cells, respectively.
As expected, a linear correlation between paste consumption and RGrid is clearly visible for all dispensed contact
fingers with a homogeneous finger cross section Af. All screen printed solar cells however suffer from an increased
grid resistance at similar paste consumption that implies that the effective silver usage is reduced by around 20%
compared to dispensed finger grids.
3.6. Influence of finger shape: Determination of effective finger width
To assess the desired optical coupling of light bouncing from a finger back onto the cell surface, we evaluated the
relative effective widths (EW) for all compared metallization technologies on a cell and module level. We defined
this value as in [13] as the fraction of effective shading and theoretical geometrical shading defined by the optical
finger width. Our EW determination method of choice was using spectrally resolved light-beam induced current
measurements (SR-LBIC). The measurement and evaluation is similar to the one recently proposed by Beutel et al.
[3]. In this work however, the reflectance of surface texture was not regarded as part of the solar cell shading.
Furthermore, EW was determined for six wavelengths from 405 nm to 1064 nm to account for spectral dependence
over the relevant range. The resulting spectrally dependent EW curve was weighed with the AM 1.5G spectrum as
well as the solar cells external quantum efficiency (EQE) for an EW value representative to standard testing
conditions. More details on this approach and the uncertainties of other EW determination methods will be
discussed elsewhere.
The results of the applied printing technologies can be seen in Fig. 9. Here, single and double screen printing was
compared with a sample with dispensed screen printing paste and one with a rheologically optimized dispensing
paste with high aspect ratios (AR).
60 80 100 120 140 160
10
20
30
40
50
60
70
80
Dispenser
Screen Printed
N = 100
GridRes(m)
mwet
(mg)
D = 50µm
D = 40µm
D = 60µm

7. Pospischil et al. / Energy Procedia 00 (2015) 000–000 7
Fig. 9 Effective finger widths on cell and module level in comparison
for different printing technologies.
Fig. 10 Comparison of finger cross sections of the four applied
technologies. Single (left) and double screen printing (right) are
depicted in the top row, the two dispensed fingers with ordinary paste
(left)and rheologically adapted paste (right) in the bottom row.
The relative effective widths on cell and module level decrease from single screen printed to double printed and
dispensed samples with high aspect ratio. In the last case, only EW = 48% of the original optical width contributes
to shading in the encapsulate case which is substantially reduced compared to EW = 72% obtained for the single
screen printed contact fingers. Assuming an ideal screen printed finger with a finger width of wo = 45µm, its
absolute effective width remains at weff = 45µm ∙ 72% = 32.4µm. Using a rheologically adapted high aspect ratio
dispensed finger with a optical width of wo = 35µm, its abs. effective width reduces to weff = 35µm ∙ 48% = 16.8µm.
At a similar number of contact fingers (here: N = 100) the effective shaded area due to contact fingers is reduced by
48% to a value of only 1.1%.
The EW of the sample with dispensed screen printing paste however remains in the range of the screen printed
samples, probably due to a severe impact of paste spreading. As a consequence, paste evaluation regarding
printability and electrical behavior can easily be performed with pastes designed for screen printing. For cell and
module production however, a rheological paste adaption leads to a substantial reduction of EW and certainly
should be taken into consideration. However, note that the EW value on the module level also depends on the
specific encapsulation materials, choice of front glass, glass coating etc. which will be subject to future studies in the
field.
3.7. Solar cell results on PERC cell structures
In order to further demonstrate applicability of dispensing technology to advanced cell concepts, a batch on
industrially preprocessed Cz p-type Si wafers with a passivated emitter and rear (PERC) cell structure was
conducted. Here, passivation on the back side was first opened locally by means of a screen printed etching paste
prior to printing Ag-pads and Aluminum. On the front side, a floating busbar concept [12] was applied to all groups
due to comparability reasons. Contact fingers of two reference groups were then applied via screen printing with an
opening width of ws = 45µm giving a total of two (i.e. dual print) and three printing steps, respectively. A third
group was then processed with dispensed contact fingers using a nozzle plate with opening width of D = 60µm. A
variation of the peak firing temperature was further conducted with all groups. In Fig. 11, IV-results of the three cell
groups at optimal firing temperature are shown, all measured in an industrial cell tester at PV-TEC.

8. 8 Pospischil et al. / Energy Procedia 00 (2015) 000–000
Fig. 11 IV-results of recent cell batch on industrial pre-processed and passivated Cz p-type material (PERC concept). All groups contain screen
printed non-contacting busbars prior to the subsequent application of contact fingers by alternatively one or two additional screen printing steps
or dispensing, respectively.
At similar contacting behaviour, the fill factor is strongly influenced by the resulting grid resistance.
Consequently, the groups with double printed and dispensed contact fingers show substantially increased FF values
than the samples with single screen printed contact fingers. As expected the jsc values of dispensed contact fingers
are increased due to substantially lower shading losses and an improved effective width. The Voc is on a reasonably
high level for all groups due to the applied PERC concept. The reason for slightly decreasing values in the double
printed and the dispensed groups has not yet been completely identified but remains in the per mill range,
comparatively. Finally, resulting cell efficiencies surpassed 20% for all groups, hence the concept of the locally
opened passivation worked well for all samples. A maximum value of η = 20.5% was reached in the dispensed
group giving an efficiency gain of 2%rel. (0.4%abs.) compared to the single screen printed group (here: dual printing)
and 1%rel. (0.2%abs.) with respect to the group with double printed contact fingers.
4. Conclusions and outlook
Fast emerging thick film printing technologies remain a dynamic challenge for any kind of alternative
metallization technology. The possibility to directly transfer results and findings from screen printing paste
development however, allows for the enhancement of dispensing technology towards industrial cell processing.
Dispensed grid lines offer a substantially more homogeneous contact shape that require 20% less silver
consumption as screen printed fingers to obtain similar grid resistances. Due to their high aspect ratio and a superior
shape, an additional benefit results from a comparison of the two technologies regarding their effective finger width.

9. Pospischil et al. / Energy Procedia 00 (2015) 000–000 9
Here, an effective width of only 48% of their optical width contributes to shading in the encapsulate case which
is substantially reduced compared to 72% obtained for screen printed contact fingers.
Consequently, on industrially preprocessed PERC solar cells, a top value of η = 20.5% was reached with
dispensing, giving an efficiency gain of 2%rel. (0.4%abs.) compared to a conventional dual screen printed reference
group. With the new developed, inline applicable dispensing platform, a continuous printing process was
demonstrated at printing speeds up to 700 mm·s-1. The integrated, advanced version of a ten nozzle parallel
dispensing print head can be easily scaled for a future application in cell production.
Acknowledgements
The authors would like to thank all co-workers at the Photovoltaic Technology Evaluation Center (PV-TEC) and
the mechanical workshop at Fraunhofer ISE for processing of the samples. This work was supported by the German
Federal Ministry for Economic Affairs and Energy within the research project “GECKO” under contract number
0325404.
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